Hypotonicity activates a lanthanide-sensitive pathway for K+ release in A6 epithelia

The nature of the pathway for K+ release activated during regulatory volume decrease (RVD) in A6 epithelia was investigated by measuring cell thickness (T-c) as an index of cell volume and by probing K+ efflux with Rb-86 as tracer for K+ (R-Rb) Cell swelling was induced by sudden reduction of basolateral osmolality (from 260 to 140 mosmol/kgH(2)O). Experiments were performed in the absence of Na+ transport. Apical R-Rb was negligible in iso- and hyposmotic conditions. On the other hand, osmotic shock increased basolateral R-Rb (R-Rb(bl)) rapidly, reaching a maximum 7 min after the peak in T-c. Quinine (0.5 mM) completely inhibited RVD and R-Rb(bl). Also verapamil (0.2 mM) impeded volume recovery considerably; lidocaine (0.2 mM) did not exert a noticeable effect. The K+ channel blocker Ba2+ (30 mM) delayed RVD but could not prevent complete volume recovery. Cs+ inhibited RVD noticeably at concentrations <40 mM. With large Cs+ concentrations (>40 mM), the initial osmometric swelling was followed by a gradual increase of T-c, suggesting activation of Cs+ influx. Chronic exposure of the basolateral surface to 0.5 mM La3+ or Gd3+ completely abolished RVD and R-Rb(bl). Acute administration of lanthanides at the time of osmolality decrease did not affect the initial phase of RVD and reduced R-Rb(bl) only slightly. Apical Gd3+ exerted an inhibitory effect on RVD and R-Rb(bl). The effect of Gd3+ should therefore be localized at an intracellular site. The role of Ca2+ entry could be excluded by failure of extracellular Ca2+ removal to inhibit volume recovery. In contrast to lanthanides, chronically and acutely administered Mg2+ (0.5 mM) inhibited RVD and R-Rb(bl) by similar to 50%. These data suggest that K+ excretion during RVD occurs through a rather poorly selective pathway that does not seem to be directly activated by membrane stretch.